Backlights are used to illuminate thick and thin film displays including liquid crystal displays (LCDs). LCDs with backlights are used in small displays for cell phones and personal digital assistants (PDA), as well as in large displays for computer monitors and televisions. Typically, the light source for the backlight includes one or more cold cathode fluorescent lamps (CCFLs). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (ELP), or one or more hot cathode fluorescent lamps (HCFLs).
The display industry is enthusiastically perusing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, require adequate idle time to ignite, and require delicate handling. LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. So, displays with LED backlights consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings. FIG. 1 shows an exemplary flat panel display 10 with a backlighting system having three independent strings of LEDs 1, 2 and 3. The first string of LEDs 1 includes seven LEDs 4, 5, 6, 7, 8, 9 and 11 discretely scattered across the display 10 and connected in series. One of ordinary skill in the art will appreciate that a LED string may have any number of LEDs ranging from one to more than twenty. The first string 1 is controlled by the drive circuit 12. The second string 2 is controlled by the drive circuit 13 and the third string 3 is controlled by the drive circuit 14. The LEDs of the LED strings 1, 2 and 3 can be connected in series by wires, traces or other connecting elements.
FIG. 2 shows another exemplary flat panel display 20 with a backlighting system having three independent strings of LEDs 21, 22 and 23. In this embodiment, the strings 21, 22 and 23 are arranged in a vertical fashion. The three strings 21, 22 and 23 are parallel to each other. The first string 21 includes 7 LEDs 24, 25, 26, 27, 28, 29 and 31 connected in series, and is controlled by the drive circuit 32. The second string 22 is controlled by the drive circuit 33 and the third string 23 is controlled by the drive circuit 34. One of ordinary skill in the art will appreciate that the LED strings can also be arranged in a horizontal fashion or in another configuration.
A critical feature for displays is the ability to control the brightness. The brightness must be sufficient, stable, and adjustable. Sufficient brightness is critical so that users can comfortably perceive the image on the display. Stability is critical because users do not want the screen to deviate from the set brightness. Adjustability is critical because users want to adjust the screen brightness to suit their preferences in given situations. In LCDs, the brightness is controlled by changing the intensity of the backlight. The intensity of the light, or luminosity, is a function of the current flowing through the light source. Therefore, the current in the backlight strings must be sufficient, stable, and adjustable. FIG. 3 shows a representative plot of luminous intensity as a function of current for an LED.
A challenge in generating sufficient light from multi-string circuits is balancing the current in each string. Strings may include different numbers of LEDs and may include different types of LEDs. Ideally, LEDs of the same type would have the same electrical properties. But in reality, light sources of the same type have different electrical properties. For example, two LEDs of the same type may have the same manufacturer specification for minimum LED voltage, i.e., the voltage difference across the LED above which current flows in the LED. In reality, the actual LED voltage for each LED can be different. The LED voltage of the LED string is equal to the sum of the individual voltages of the LEDs in the string connected in series. Therefore, the LED voltages of LED strings can vary significantly from each other depending on the number and types of LEDs in the strings. Different LED voltages also mean different voltage drops across each string and different currents flowing through each string. Also, a string may contain one or more burnt out or damaged LEDs, causing short circuit conditions. LED temperature and the LED string current can affect the actual voltage across the LED. All these factors influence the amount of current that is required to flow through the string to generate a desired luminosity.
FIG. 4 illustrates an exemplary technique of implementing LED strings in a display 40. A power supply 41 is shown connected to strings 42, 43 and 44. The strings 42, 43 and 44 are connected to the ground. The power supply 41 supplies input voltage Vin to the strings 42, 43 and 44. String 42 includes seven LEDs 45, 46, 47, 48, 49, 51 and 52. If the voltage drops across the LED strings 42, 43 and 44 are different from each other, the current in each string 42, 43 and 44 will be different for a given input voltage Vin. The challenge in designing multi-string circuits is to ensure that all the light sources are illuminated and that the current in each string is sufficient to bring the light source to the appropriate intensity.
One method for assuring sufficient current in each string 42, 43 and 44 is to ensure that the input voltage Vin is sufficiently high to induce the target current in the LED string having the highest voltage drop 42, 43 or 44. This method assumes that the voltage drop in each string 42, 43 and 44 is such that it does not cause the Vin to exceed the LED manufactures' specifications. Users of this method typically increase this input voltage Vin further to assure sufficient current flows through the strings 42, 43 and 44 in case of any discrepancies in the manufactures' specifications. However, as a result, each string 42, 43 or 44 generates more than the needed current. That is inefficient because it generates unneeded light and dissipates more current. This method might require the usage of guard banding and other techniques to prevent the damage that may result to the circuit components from the dissipation of excess power.
FIG. 5 illustrates a prior art technique for balancing LED strings in the display 50. A power supply 51 is shown coupled to one of the ends of the LED strings 52, 53 and 54. The power supply 51 provides driving voltage VOUT to the LED strings 52, 53 and 54. The other ends of the LED strings 52, 53 and 54 are connected to the drains of the field effect transistors (FETs) FET1, FET2 and FET3. The sources of the field effect transistors FET1, FET2 and FET3 are coupled to the ground by way of the resistors R1, R2 and R3 respectively. The currents flowing through the LED strings 52, 53 and 54 are sensed by the resistors R1, R2 and R3 respectively, and the sensed voltages are provided as feedback inputs to comparators, for example, error amplifiers EA1, EA2 and EA3 respectively.
The other inputs to the error amplifiers EA1, EA2 and EA3 include reference voltage sources 55, 56 and 57 for providing reference voltages VREF1, VREF2 and VREF3. The outputs of the error amplifiers EA1, EA2 and EA3 are coupled to the gates of the transistors FET1, FET2 and FET3 to control the current flow through the LED strings 52, 53 and 54. One of ordinary skill in the art will appreciate that the voltages applied to the gates of the transistors FET1, FET2 and FET3 control the on and off times of the transistors FET1, FET2 and FET3, and also control the maximum rate of current flow through the transistors FET1, FET2 and FET3.
One of ordinary skill in the art will also appreciate that the reference voltages VREF1, VREF2 and VREF3 represent the desired current flows through the LED strings 52, 53 and 54 respectively. The error amplifiers EA1, EA2 and EA3 compare the reference voltages VREF1, VREF2 and VREF3 respectively with the sensed feedback currents for the strings 52, 53 and 54 respectively and provide compensating control voltages to the gates of the transistors FET1, FET2 and FET3 respectively, to ensure that that the desired currents flow through the LED strings 52, 53 and 54.
The display 50 of FIG. 5 has many drawbacks including that it requires extra comparison circuitry and requires that the same driving voltage VOUT is continuously provided to all the LED strings 52, 53 and 54, regardless of the current flow requirement of a particular string 52, 53 or 54. The present invention provides innovative systems and methods for high efficiency automatic power balancing of LED strings used for backlighting electronic displays.